Field of the Invention
The present invention relates to a display apparatus and a method to determine a format.
Description of the Related Art
Display apparatuses such as liquid crystal display apparatuses drive a display unit such as a liquid crystal panel with a signal obtained by converting an analog image signal, which is output from an image output apparatus such as a personal computer (PC), into a digital image signal through an A/D conversion. In performing the A/D conversion, it is important to perform the sampling of an image signal based on an appropriate sampling clock for digitization. However, if the sampling is performed based on a false sampling clock, there is a likelihood of image degradation such as image irregularities and image deviations occurring. As a method for performing the sampling with an appropriate sampling clock, signal parameter values such as the frequency and the polarity of a synchronization signal contained in the image signal may be used. Specifically, the signal parameter values such as the frequency of the synchronization signal detected from the input image signal are identified with signal parameter information items on various image signals stored in a format table retained by an apparatus to determine the format of the input image signal. Then, the sampling is performed based on a sampling clock defined in the determined format.
In recent years, many types of image signal formats have been available with the diversification of formats supported by display apparatuses. Therefore, since there are formats having signal parameter values similar to signal parameter values detected from an input image signal, it may be difficult to exactly determine a format. If a format is falsely determined, the image signal is sampled with a false sampling clock and thus an appropriate A/D conversion is not allowed.
Japanese Patent Application Laid-open No. 2008-107474 discloses a method for assigning similarity degrees, which become lower as the differences between respective signal parameter values detected from an input image signal and signal parameter values stored in a format table become larger, to the respective signal parameter values and determining a format based on the sum of the similarity degrees. In addition, Japanese Patent Application Laid-open No. 2008-224773 discloses a method for comparing a vertical frequency and a data enable (DE) period of an input image signal with signal parameter values stored in a format table and determining a format based on the comparison results.
As in the related arts described above, it is possible to compare the measurement values of signal parameter values such as a synchronization signal frequency, a blanking period, a DE period, and a total number of vertical lines of an input image signal with signal parameter values stored in a format table to determine a format.
Here, since there is no analog image signal showing a DE period, the level of an image signal is measured to estimate the DE period. However, if an image nearly in black or an image in which all the effective display areas except for some areas are in black is input, a DE period may not be exactly estimated. Therefore, according to a configuration in which the detection result of the DE period of an input image signal is compared with values stored in a format table to determine a format, there is a likelihood that a format is falsely determined.
For example, in the case of a multi monitor configuration in which the image output of a notebook PC is input to an outside display apparatus so as to be displayed on multiple monitors, the outside display apparatus may have a larger pixel number than the display unit of the notebook PC. In such a case, an image, which is obtained by adding a black frame area to the periphery of an image (desktop image) output to the display unit of the notebook PC, is likely to be output to the outside display apparatus. Here, it is assumed that the image output to the display unit of the notebook PC represents an image not at a zero level (black) (for example, a white image). In this case, the periphery of the image not in black is surrounded by a black area in the image input to the outside display apparatus. Therefore, a period corresponding to the surrounding black image becomes zero (pedestal level) as a signal level. Accordingly, if the signal is measured, only the desktop image region is measured as a DE period while the surrounding black image region is not measured as a DE period although originally showing the DE period. If the measurement result is identified with a format table to determine a format, there is a likelihood that a format is falsely determined. Therefore, if the sampling of an image signal, the adjustment of a display field angle, or the like is performed based on signal parameter values defined in the determined format, image irregularities such as the smearing and the blurring of an image and the deviation of the display field angle are likely to occur.